CAVITY WALLS AND ITS CONSTRUCTION

A cavity wall or hollow wall is the one which consists of two separate walls, called leaves or skins, with a cavity or gap in between. The two leaves of a cavity wall may be of equal thickness if it is a non-load bearing wall, or the internal leaf may be thicker than the external leaf, to meet the structural requirements. The two portions of the wall may be connected together by metal pins or bonding bricks at suitable interval.  It also prevents the dampness to enter and acts as sound insulation. Thus they are normally the outer walls of the building. The size of cavity varies from 4 to 10 cm. The inner and outer skins should not be less than 10 cm each (half brick). 

Figure 1. Cavity wall

Advantages

Cavity walls have the following advantages over other walls:

1. There is no direct contact between the inner and outer leaves of the wall (except at the wall ties). Hence, the external moisture (dampness) cannot travel inside the building.
2. The cavity between the two leaves is full of air which is bad conductor of heat. Hence, transmission of heat from external face to the inside the room is very much reduced. Cavity walls have about 25% greater insulating value than the solid walls.
3. Cavity walls also offer good insulation against sound.
4. The nuisance of efflorescence is also very much reduced.
5. They are cheaper and economical.
6. Loads on foundations are reduced because of lesser solid thickness.

Construction

Generally, the cavity wall is set centrally over the concrete base, without any footings. According to I.S. recommendations, the lower portion of the cavity may be filled with lean concrete upto few centimeters above the existing ground level. The top of the filling should be sloped (Figure 2) with weep holes at 1 m intervals along the outer leaf of the wall. The inner leaf may be of common bricks and the outer leaf with any designed kind of facing bricks or it may also be common bricks finished with rendering. The two leaves should be tie together with wall ties.

Figure 2. Position of cavity at foundation level

Bonds for cavity wall construction should consists of stretcher bond for half brick leaves and any ordinary bond, such as English bond or Flemish bond for leaves which are one brick or more in thickness. Where solid walls are joining cavity walls, bonding of former into the latter should conform to the principle shown in figure 3. Stretchers in the solid wall should extend half brick into the inner leaf of the cavity wall and closers as shall be used for good bonding.

Figure 3. Junction between Solid Wall and Cavity Wall

Bricks should be lad very carefully to leave the cavity free from mortar droppings. Two leaves of the wall should be raised simultaneously and uniformly. The position of wall ties should be predetermined so as to have uniform spacing preferably in centres. The cavity should be made free from rubbish and mortar droppings by means of a timber batten 25 mm thick and width about 12 mm less than the cavity, resting over the ties. The battens may be lifted by means of wires or rails attached to the battens, as shown in figure 4. The batten is supported on wall ties and the brick work is carried out on either side of the batten, to the height where next row of wall ties are to be provided. After this, the batten is lifted up, cleaned of mortar droppings and replaced over the next row of wall ties.

Figure 4. Cavity Wall Construction